1. Field of the Invention
The present invention relates to measuring web tension, and in particular for the sonic measurement of web tension in paper machines.
2. Description of Prior Art
As is well known to those skilled in the paper making art, and to those skilled in the handling of moving webs of paper, excessive tension can tear the web and inadequate tension can cause edge flutter at high machine speeds, which will eventually cause a break if the amplitude of the flutter becomes excessive. In both cases, down time results. Conventionally, unsupported web tension is controlled by slight differences in machine speed, and machine speed differential is manually controlled and must be changed as the web shrinks as it passes through different sections of the machine. Therefore, the control of tension through the control of machine speed has heretofore been a critical operation and must be frequently monitored.
An open draw is necessary because as the web loses water, it shrinks. Shrinkage in the machine direction is restrained somewhat by the changes of machine speeds. Therefore, the paper quality, for example, ultimate strength, stretchability, etc., will be affected by the web tension.
As reported by K. W. Britt, Pulp and Paper Technology, 2nd, E. Van Nostrant, 1970, p.468 "There is no known instance of a successful attempt being made to automatize any of the critical draws on a paper machine. For example, on an open draw machine, the speed difference between the machine wire and the first press felt has an important effect on both machine runability and the product of mechanical and/or functional properties. A technique for sensing the tension in such a draw and adjusting it to maintain web tension at a specified level would be quite beneficial. At present, no practical method for sensing said tension is available, but optical scanning methods and knowledge of paper rheology (as a function of moisture content) may offer a route to do a successful solution."
Transverse waves are created when a string, under tension, is disturbed, and transverse waves are created when the surface of water is disturbed. Likewise, when a membrane under tension is disturbed transverse waves are also generated. If the bending stiffness of the membrane can be neglected, these transverse waves have a wave velocity in accordance with the relationship ##EQU1## where T=tension in membrane (lb force/ft),
w=weight of membrane/unit area (lb mass/ft.sup.2), PA1 a=wave velocity (ft/sec), PA1 g.sub.c =conversion factor (37.17 lb mass/lb force ft sec.sup.2)
In U.S. Pat. No. 4,109,520, Leif Eriksson discloses a method for measuring web tension of a stationary web which applies a frequency near the resonance of a loudspeaker pressed against the web. The impedance of the loudspeaker at this frequency is a function of the web tension.
In U.S. Pat. No. 2,661,714, I. A. Greenwood, Jr. et al disclose a method of measuring web thickness of a traveling web with ultrasonic techniques in which the web must be pressed or held against an anvil-type support at the measuring location below an X-cut piezoelectric crystal which produces air compressional waves.